Lithographic printing is based on the principle that oil and water do not mix. A lithographic printing plate has non-image areas which absorb water (hydrophilic areas) and image areas which repel water (hydrophobic areas). For wet offset lithographic printing, the plate is first dampened by water so that the ink is applied to the wetted printing plate to form the image. The ink, which is inherently oily, is rejected by the wet areas and adheres to the image areas. The ink from the inked printing plate is then transferred or “offset” to a rubber blanket. Next, the image on the blanket is transferred to the substrate, mostly paper, to produce the printed product.
The process of imaging the lithographic printing plate usually involves wet processing and use of unhealthy chemicals and high cost imaging devices such as Computer to Plate (CTP). There is a need for a printing member that can be imaged on demand at the print shop in a simple chemistry-free process and using an inexpensive imaging engine like inkjet printers.
The application of inkjet printers for producing offset printing plates is an issue which has attracted digital printing field developers due to its potential for inexpensive imaging hardware and workflow. In many applications utilizing inkjet printer engines, the solutions employ the use of special inkjet fluid instead of original inks.
A few examples include Japanese patent JP56105960 issued to Nakayama et al. which describes the use of heat activated hardening material in oleophilic inkjet inks, forming the image areas on hydrophilic substrates. The substrate may or may not be coated. Also, U.S. Pat. No. 4,833,486 by Zerillo utilizes a hydrophobic wax based inkjet ink. U.S. Pat. No. 5,738,013 by Kellet describes an ink-jet plate-making process involving the use of reactive inkjet ink, which is bonded to the litho plate by a chemical reaction activated by radiant energy. U.S. Pat. No. 5,820,932 by Hallman et. al describes a variety of reactive inks and processes. U.S. Pat. No. 6,184,267 by Osawa et al. describes oil-based ink for preparing printing plate. U.S. application 2006/0223006 by Shimada et al. describes reactive inkjet composition, being jetting on to the reactive coating forming ink accepting image.
Still, other examples include U.S. Pat. No. 5,495,803 which describes a solid or phase change type of ink jet printing to form a photomask for a printing plate.
U.S. Pat. No. 5,511,477 discloses a method for the production of photopolymeric relief-type printing plates comprising the formation of a positive or a negative image on a substrate by ink jet printing with a photopolymeric ink composition, optionally preheated to a temperature of about 30.degree.-260.degree. C., and subjecting the resulting printed substrate to UV radiation, thereby curing the ink composition forming the image.
U.S. Pat. No. 5,312,654 discloses a method for making lithographic printing plates comprising the formation of an image on a substrate having an ink absorbing layer and a hydrophilized layer between the substrate and the absorbing layer by ink jet printing using a photopolymerizable ink composition, and exposing it to an actinic light in the wavelength region with which said ink composition is sensitized to cure the image.
In U.S. Pat. No. 6,455,132 a fluid composition is described comprising a copolymer, having a plurality of tertiary amine sites selected from the group consisting of polyacrylates, polyamides, styrenated polyacrylates and polyurethanes, which can be applied by an ink jet printing apparatus.
In EP-A 1157825 a method is disclosed for the preparation of a lithographic printing plate, comprising dispensing information-wise by means of ink jet printing droplets of a fluid onto a surface of a lithographic receiver, characterized in that said fluid contains an oleophilizing agent having in its chemical structure a phosphorous containing group capable of reacting with said surface of said lithographic receiver.
Typical applications that provide imaging with inkjet printers require special inkjet printers. For example, such special inkjet printers include solvent base or wax thermal jet printers. These printers must be suitable for the special solvent, photo-curable or hot melt inks that are utilized. These specialized printers are considerably more expensive than standard inkjet printers and are not as environmentally friendly as the inkjet printers that utilize water based inks. An additional disadvantage of such special inkjet printers is that chemical post-processing needs to be performed for such typical applications.
Therefore, the use of water base inkjets for the imaging of offset plates is generally a more attractive solution from economical and ecological points of view.
Additional references related to this field of art include Szumla, DeBoer et al., Eastman Kodak (U.S. Pat. No. 6,758,140 and application No 20040051768), Figov et al., Creo IL (U.S. Pat. No. 7,044,053) and Porat et al., VIM Technologies Ltd. (U.S. Pat. No. 8,062,720). These references describe direct inkjet printer image-able lithographic plates which can be imaged using standard printers and standard water based pigment ink. Such plates do not require chemical development but rather, simply require baking of the imaged plates at temperatures of about 130-150 C prior to installing on the printing plates. These technologies require special baking system, which increase the investment in plate imaging equipment and add post imaging operation and press-ready time.